A03: Shockless explosion combustion

Summary

TP A03 develops the “shockless explosion combustor” (SEC). As a Rijke-tube driven by quasi-homogeneous autoignitions, the SEC realises approximately isochoric combustion with shock-free pulsations and without losses from deflagration acceleration. In Phase I the realizeability of sufficiently homogeneous autoignitions was demonstrated. To also achieve the necessary acoustic-chemical resonance with available fuels, a full-fledged SEC will be embedded in a high-pressure chamber in Phase II. Advanced issues, e.g., process stability or tunability from idle to full power output will again be investigated numerically.

2nd Funding period 2016 – 2020

Resonant operation

The aim of the pulsed shockless explosion combustion chamber is the generation of a pressure-increasing combustion by quasi-homogeneous self-ignition of the mixture in the combustion chamber. Since the combustion chamber is filled axially, the residence time of the mixture in the combustion chamber is not constant. This is compensated by a variation of the ignition delay time, so that a constant ignition time is achieved in the entire combustion chamber. For this purpose, the equivalence ratio is varied over the duration of injection. A pressure wave generated during ignition passes through the tube and is reflected at the open end as a suction wave, which then serves to refill the tube with an ignitable mixture.

Optimization of the injection at the atmospheric test bench

For the targeted control of the equivalence ratio, a precise injection of the required fuel curve is necessary. In the first funding period, a test rig for the investigation of the injection and its control at atmospheric pressure conditions was set up. To minimize the ignition delay time of the dimethylether-air mixture, the air is preheated up to 700 ° C. In order to allow the observation of the auto-ignition in the combustion chamber, the flow is stopped, and the inflowing preheated air is deflected via a bypass.

In the second funding period, the heat loss between the air preheater and the combustion chamber was reduced, whereby higher temperatures in the combustion chamber and thus a lower ignition delay time could be realized. This made the stopping of the flow and the use of a bypass unnecessary. The convection of the mixture during the ignition delay time is compensated by a straight tube. This novel 'single tube' design of the SEC is very similar to the design of the SEC in resonant mode and allows investigations that are essential for the design of the SEC at elevated pressure.

With the new configuration of the atmospheric SEC test rig, the quantification of the new injection geometry was first made, whereby the most important parameters for the realization of a precise injection could be determined. Further experimental investigations should show that the controllability of the ignition behavior can be improved by an optimized injection geometry. In addition, the dependence between ignition behavior and resulting pressure increase is to be determined.

Pressure increase

In order to operate the SEC in resonant mode, a further reduction of the ignition delay times is necessary, which can be achieved by an increase in pressure. In order to increase the pressure in the combustion chamber while ensuring an acoustically open end for generation of a suction wave, a ball plenum was designed, which is connected to the combustion chamber. This new test rig is intended to show that the resonant operation of the SEC is technically feasible.

Software

The software, which was developed at Freie Universität Berlin to simulate the SEC and the pulse detonation combustion (PDC) in quasi-1D, was expanded and refined during the second funding period. It was adapted to the demands of the users, including those from other subprojects. An adaptive grid refinement as well as the possibility to model more complex systems by coupling quasi-one-dimensional simulations were implemented. Among other things, the latter provides insight into the interaction of any number of combustion tubes with each other or with a greatly simplified turbine via a toroidal plenum. Different configurations with respect to the radius and the length of the plenum, the distribution of the combustion tubes over the plenum length and their angle of incidence can be investigated. But above all, the fire sequence has so far been the focus of interest and the possibility to restart a combustion tube after a misfire using the other combustion tubes. These interactions will also be analyzed by means of nonlinear acoustics. One of the software enhancements planned for future is the inclusion of diffusion. This will allow the simulation of turbulence effects together with already completed extensions. The basis for this is the one-dimensional turbulence model, which is used to simulate turbulence in a time-saving, phenomenological manner.

1st Funding period 2012 - 2016

Summary

The Shockless Explosion Combustion (SEC) is a new proposed way to realize constant volume combustion in gas turbine combustion systems. As for the pulsed detonation combustion (PDC) in Project TP A01 the SEC is expected to increase the overall efficiency significantly. In contrast to the PDC this increase is realized without shockwaves and thus reduces the loads on the machine. The goal of Project TP A03 is to realize the SEC-cycle in a laboratory scale. It is divided in two parts:

The experimental part of the project is located at the Technische Universität Berlin. The Chair of Fluid Dynamics is conducting mixing tests in a water channel and single ignition tests in the new laboratory. These tests are used to realize a working SEC-Combustor in the laboratory at the end of the project.

The numerical part of the project is located at the Freie Universität Berlin. Using one-dimensional numerical models of the process the theoretical optimum for the mixing layering and the sensitivity of the process is investigated for different fuels. The goal is to identify a fuel injection curve that assures a reliable and quasi homogeneous autoignition for the SEC-process. In Addition numerical approaches for the simulation of thermally perfectly mixed gases employing the Euler-equations are investigated.

The analysis of the results from project A07 showed that fuels suitable for the SEC are only available at high pressures. Instead of the full cycle, this project's experiment therefore aims to show the technical realizability of a homogeneous ignition in a controlled environment. Fluidics are used to control the process. The results from this experiment will be used to pursue a high pressure test rig in the next project phase. Since technical constraints only affect the experiment, the theoretical part of this project concerns itself with the preliminary investigation of the initiation of the SEC from the classical turbine cycle.

Publications

Berndt, P.: On the Use of the HLL-Scheme for the Simulation of the Multi-Species Euler Equations. Springer Proceedings in Mathematics & Statistics, Finite Volumes for Complex Applications VII: Elliptic, Parabolic and Hyperbolic Problems:809–816, 2014.